1. Field of the Invention
The present invention relates to an improved retaining structure for a rolling element and more particularly to a structure that ensures the rolling element rolls stably and smoothly so as to provide a high precision operation of linear guide.
2. Description of the Prior Art
A retaining for rolling elements is usually used to contain rolling elements so that the rolling elements can achieve an excepted rolling and further perform repeated circumrotation motions. It is often applied to high-speed operations or accuracy adjustments, such as for a linear guide.
Please refer to FIG. 1 and FIG. 2, which are respectively a perspective schematic view and a top view of a retaining for a rolling element of the prior art. The retaining comprises a plurality of separated pieces 1a, two pull-resisting ribbons 3a, and a plurality of rolling elements 4a. Two sides of the separated pieces 1a both connect with a connecting ribbon 2a and the two connecting ribbons 2a are connected with the two pull-resisting ribbons 3a on two sides of the separated pieces 1a. Thereby, the separated pieces 1a are equidistantly arranged between the two pull-resisting ribbons 3a. 
A front surface and a rear surface of the separated pieces 1a both have an integrally formed spherical surface 11a. The spherical surfaces 11a are relatively formed with the rolling elements 4a wherein the rolling elements 4a are rolling balls. The rolling elements 4a can be contained between the separated pieces 1a. The spherical surfaces 11a are closely adjoined to the rolling elements 4a. Thereby, the rolling elements 4a are stably rolled between the separated pieces 1a. 
Please refer to FIG. 3 and FIG. 4, which are separately a perspective schematic view and a top view of a retaining for a rolling element of another prior art. A front surface and a rear surface of the separated pieces 1a both have a groove 12a. The grooves 12a are formed by concaving end surfaces of the separated pieces 1a. Two side edges of the grooves 12a provide a point contact to surfaces of the rolling elements 4a. The rolling elements 4a are disposed between the separated pieces 1a in a manner such that they can roll.
Each of the pull-resisting ribbons 3a has a flexible section 31a and a non-flexible section 32a thereon. The flexible section 31a is disposed at a longitudinal distance between two separated pieces 1a that correspond to the pull-resisting ribbon 3a, and the pull-resisting ribbon 3a has a non-flexible section thereon 32a. The non-flexible section 32a is disposed at a longitudinal distance to the sides of the separated pieces 1a and near to a front edge and a rear edge of the separated pieces 1a that corresponds to the pull-resisting ribbon 3a. 
The rolling elements 4a are rolled and kept between the separated pieces 1a to perform repeated circumrotation motions according to combination of the above-mentioned elements. When the rolling elements 4a pass through circumrotation areas, the flexible sections 31a of the two pull-resisting ribbons 3a perform certain elasticity deformation to reduce the friction so that the rolling elements 4a fluently pass the circumrotating areas and continuously perform repeated circumrotation motions.
However, the above-mentioned retaining for rolling elements has the following drawbacks in practice:
1. The rolling elements 4a are respectively wrapped by a single spherical surface 11a of the separated pieces 1a and matched in a surface contacting manner so that the rolling elements 4a produce an inappropriate friction and are inappropriately clipped when performing repeated circumrotation motions. In the second prior art, the rolling elements 4a are matched with the grooves 12a in a point contacting manner so that the rolling elements 4a cannot be stably kept in a central position of the separated pieces 1a causing the rolling elements 4a to escape when performing repeated circumrotation motions.
2. In order to increase the strength of the connection between the connecting ribbons 2a and the pull-resisting ribbons 3a, a collective connection area is added. However, this reduces the uniform flexibility of the flexible sections 31a so as to produce an inappropriate friction and deformation that influences the repeating circumrotation motion. In another prior art, there is a longer flexible section and a smaller collective connection area that produces insufficient strength in the structure so as to fracture the repeating circumrotation connection.
3. The molds of the spherical surfaces 11a of the separated pieces 1a (as shown in FIG. 1) or the grooves 12a (as shown in FIG. 2) are not easily released and easily distort the size of the spherical surfaces 11a and the grooves 12a. 
The inventor of the present invention recognizes the above shortages should be corrected and special effort has been paid to research this field. The present invention is presented with reasonable design and good effect to resolve the above problems.